Therefore, if halos result from the α-decay of 210Po to 206Pb, their appearance
should resemble the idealized schematic
(Fig. 1b), and the light and dark halos of
this type in biotite should exhibit radius
variations consistent with the differences
between lower and higher coloration band
sizes (Table 1, columns 2, 3, 6, 14, and 15).
Further, such halos, whether very light or
very dark, should appear without any outer
ring structure, as illustrated in Fig. 1n.
Compare also the densitometer profiles of
the halo negatives of Fig. 1f (the U halo)
and Fig. 1n shown in Fig. 2b and Fig. 2, c to
e, respectively. Fig. 1o shows three similar
halos in fluorite; here, irrespective of
coloration differences, the halo radii are the
same and correspond to the Eα of 210Po
(Table 1,
columns 4, 6, and 20). Accordingly, the
halos in Fig. 1, n and o, are designated
210Po halos. (Actually I should emphasize
that since not all biotites exhibit the same
coloration responses, the radius
measurements in Table 1 are strictly valid
only for the particular micas I used. I did
try to illustrate a range of responses by
utilizing four different biotites for the U
halo and the three Po halo types.)

By analogy, the moderately developed
biotite halo in Fig. 1p shows a marked
resemblance to the idealized halo that
would form from the sequential α-decay of
214Po and 210Po (see Fig. 1c).
Table 1,
columns 2, 3, 6, 7, 16, and 17, shows the
correspondence of the radii with
band sizes. The prominent
unmistakable feature of the
214Po halo is the broad
annulus separating the inner
and outer rings [see the
densitometer profile of Fig.
1p shown in Fig. 2f and
figures 7 to 9 in (6)]. With
respect to comments in (11) it
should be noted that the
214Po halo can easily be
distinguished from a U halo.

The last correspondence to be established
is the resemblance of the two three-ring
halos in biotite (Fig. 1q) and two similar
halos in fluorite (Fig. 1r) to the idealized 218Po
halo (Fig. 1d) showing the ring
structure from the sequential α-decay of
218Po 214Po, and 210Po. In biotite such halos
may appear very light to very dark with
radii correspondingly slightly lower and
higher (excluding reversal effects) than
those measured for medium coloration
bands (compare Table 1, columns 2, 3, 18,
and 19). Cursory examination of inferior
specimens of this halo type could lead
to confusion with the U halo, especially in
biotite, where ring sizes vary
slightly because of dose and
other effects. However, good specimens of
this type are easily distinguished from U
halos, even in biotite. In fluorite, where the
ring detail is better, a most important
difference between 238U and 218Po halos is
delineated, that is, the presence of the 222Rn
ring in the U halo (Fig. 1a) in contrast to its
absence in the 218Po halo (Fig. 1d). For
example, note the slightly wider annulus
(3.9 μm) between the 210Po and 218Po rings
of the 218Po halo compared to the
equivalent annulus (3.0 μm) in the 238U halo
(Fig. 1, a, d, h, h', r, and r'). This is
evidence that the 218Po halo indeed initiated
with 218Po rather than with 222Rn or any
other α-decay precursor in the U chain. As
further proof, Table 1 (columns 4, 11, 12,
and 21 shows that the 218Po halo radii agree
very well with equivalent band sizes and U
halo radii in this mineral. Additional Po
halo types also exist (3) but are quite rare.
[As yet I have found no halos at all in
meteorites or lunar rocks (19)].

The preceding discussion
has shown [p. 243]
that Po halos can be positively identified by
ring structure studies alone. That x-ray
fluorescence analyses also provide quite
convincing evidence is seen in Fig. 3c,
where I show for the first time the x-ray
spectra of a Po halo radiocenter
(specifically, a 218Po halo). Comparison of
Fig. 3, b and c, reveals that the Pb in the Po
halo radiocenter in fluorite did not arise
from in situ decay of U. [Longer runs have
shown small amounts as Se as well as U in
some Po halo radiocenters (18).] On the
other hand, the presence of Pb is to be
expected in a 218Po halo radiocenter because
the decay product is 206Pb. That the parent
nuclide was 218Po and not a β-decaying
isomer precursor (13, 20) follows from
half-life considerations of the U halo U/Pb
ratio (> 10); the proposed isomer, if formed
at nucleosynthesis, should now be
detectable in Po halo radiocenters. No trace
of this isomer has yet been found, and I
thus view the isomer hypothesis as
untenable.